Adjustable range reference signal circuit



Aug. 1966 H. L. LATHAM, JR 3,270,275

ADJUSTABLE RANGE REFERENCE SIGNAL CIRCUIT Filed Dec. 21, 1962 REGULATEDPOWER SUPPLY INVENTOR. HARRY L. LATHAM, JR.

ATTORNEY. 4

United States Patent I 3,270,275 ADJUSTABLE RANGE REFERENCE SIGNALCIRCUIT Harry L. Latham, Jr., Philadelphia, Pa., assignor to Honeywellllnc., a corporation of Delaware Filed Dec. 21, 1962, Ser. No. 246,579 8Claims. (Cl. 323-79) This invention relates to electrical apparatus and,more particularly to a reference signal range changing circuit.

In the art relating to electronic industrial control systems there isfrequently provided a source of reference signals against which othersignals may be compared. It is desirable that the reference source beadjustable to provide a predetermined but variable reference signalagainst which the other signal may be compared. It is also desirablethat the adjustability of the reference source signals be such that thesignals are variable in a plurality of step-wise range increments and,further, be continuously variable within each of the range increments.

It is accordingly an object of the present invention to provide animproved reference signal circuit which is adjustable in a plurality ofincremental ranges and, further, is continuously adjustable within eachof said ranges.

It is a further object of the present invention to provide a referencesignal source circuit which is characterized in an avoidance ofdiscontinuities or providing a bumpless transition between successiveranges.

It is a further object of the present invention to provide an improvedreference signal source circuit as set forth which is furthercharacterized in that the bumpless transition between successive rangesis operative whether the circuit is being adjusted to provide for anincreasing reference signal or decreasing reference signal.

In accomplishing these and other objects there has been provided, inaccordance with the present invention, an improved reference signalsource wherein a plurality of range determining resistors are seriallyconnected across the output terminals of a regulated power supply. Aswitching arrangement is provided for switching between successiveranges represented by the serially connected range resistors. There isalso provided a slidewire arrangement for continuously adjusting the magnitude of the reference signal within each of the selective ranges. Therange switching elements are so arranged that between each of theoperating range positions on the switched elements there is provided ahold position which provides a by-pass for the slideware adjustment.This permits the slidewire to be returned to its initial position beforeadjusting the range switch to the next operating range position. Thisarrangement obviates the discontinuity in the developed reference outputsignal which would otherwise be produced by switching to the next rangeposition of the switches while the slidewire element is at the upperlimit of its travel.

A better understanding of this invention may be had from the followingdetailed description when read in connection with the accompanyingdrawing in which the single figure is a schematic circuit representationof apparatus constructed in accordance with the present invention.

Referring now to the drawing in more detail, there is shown in thesingle figure a regulated power supply 2 connected to a source ofalternating current energy 4. The regulated power supply itself does notform a part of the present invention, and consequently, it is shown inblock form. Any one of numerous well-known regulated power supplycircuits may be utilized to provide the 3,270,275 Patented August 30,1966 regulated output signal. In fact, the output of the regulated powersupply may be either an alternating current signal or a direct currentsignal, as desired. The regulated output signal from the power supplycircuit 2 is applied between a pair of leads 6 and 8, respectively.Between the leads 6 and 8 there is connected a series array ofrange-determining resistors 10, 12, 14 and 16, respectively. In theillustrated embodiment of this invention, means are provided forestablishing three distinct ranges for the reference signal. In order toestablish these ranges a plurality of connections are made to thejunctions between the several range determining resistors. For example,a first connecting lead '18 is connected to the junction betweenresistors 10 and 12, a second connecting lead 20 is connected to thejunction between the resistors 12 and 14, and a third connecting lead 22is connected to the junction between the resistors 14 and 16.

In order to effect the selection between the several ranges there areprovided three multi-position switches, 24, 26 and 28, respectively. Thefirst switch 24- is illustrated as having five fixed contacts 24a-e anda movable contact 24 Similarly, the second switch 26 is illustrated ashaving five fixed contacts 26ae and a movable contact 26 Again, thethird switch 28 is shown as having five fixed contacts 28a-e and amovable contact 28 The movable contact members of these three switchesare interconnected or ganged together for simultaneous operation betweensuccessive contact points. This interconnection is represented in thedrawing by the dotted line 30.

The first two fixed contacts 24a and 24b of the switch 24 are connectedtogether and to the connecting lead 18. The connecting lead 18 is, inturn, connected to the junction between the range determining resistors10 and 12. The third and fourth contacts 24c and 24d of the switch 24are connected together and to the connecting lead 20 which is, in turn,connected to the junction between the range determining resistors 12 and14. The fifth contact 24e of the switch 24 is connected to theconnecting lead 22 which is, in turn, connected to the junction betweenthe range determining resistors 14 and 16. The first and second fixedcontacts 26a and 26b of the second switch 26 are connected together andto the connecting lead 20. The third and fourth fixed contacts 26c and26d of the switch 26 are connected together and to the connecting lead22. The fifth contact 26e of the switch 26 is connected directly to thelead 8. The movable contact 24 of the switch 24 is connected to one endof the terminal of a slidewire resistor 32. The movable contact 26 ofthe switch 26 is connected to the opposite end terminal of the slidewireresistor 32. In the third switch 28 the first fixed contact 28a, thethird fixed contact 280, and the fifth fixed contact 2812 are connectedtogether and to the slider 34 which, in turn, operates along theslidewire resistor 32. The second fixed contact 28!) and the fourthfixed contact 28d of the switch 28 are connected together and to themovable contact 26f of the switch 26. The movable cont-act 28 of theswitch 28 is connected to an output voltage dividing circuit including afirst resistor 36 and a second resistor 38 connected in series, thelower end of the resistor 38 being connected to the lead 8. A pair ofoutput terminals 40 and 42 are connected, respectively, to opposite endsof the resistor 38.

In operation the regulated power supply 2 may be selected to provide anypredetermined output voltage desired across the leads 6 and 8. Typicallythis voltage may be on the order of 6.3 volts. This voltage is thenapplied across the series string of the range determining resistors 10,12, 14 and 16. If the resistors 10, 12, 14

and 16 are of equal value, then the applied voltage will be uniformlydivided and the corresponding sub-multiple of the applied voltage wouldappear at each of the junctions between the aforesaid range determiningresistors. If on the other hand, it is deemed desirable to provideunequal sub-divisions of the applied voltage at the several junctions,then, of course, the relative values of the range determining resistorsmay he of unequal value. For the sake of simplicity in description itwill be assumed that the resistors 10, 12, 14 and 16 are of equal value.With this arrangement it may be seen, assuming the aforementioned outputvoltage from the regulated power supply 2, that slightly less than 1.6volts will be developed across each of the 4 resistors. Initially let ithe assumed that the movable contact on each of the three switches 24, 26and 28 is at its lower most position-that is, in engagement with thecorresponding fixed contact e in each of the switches. Let it further beassumed that the slider 34 on the slidewire 32 is at its lower mostposition as viewed in the drawing. Under these conditions the contact 24of the switch 24 is connected through the fixed contact Me to the lead22.. This lead, in turn, is connected to the junction between resistors14 and 16. The movable contact 26 is connected through the fixed contact26c to the system common lead 8. With the switches 24 and 26 thuspositioned, the difierence in potential between the common lead 8 andthe connecting lead 22 is applied across the end terminals of theslidewire resistor 32. With the slider 34 at the lowest end of theslidewire resistor 32 the potential on the slider 34 will be zero withrespect to the common lead 8. The movalble con tact 28] of the switch 28is connected through the fixed contact 28a to the slider 34. Thus, underthese conditions, the potential across the voltage dividing resistors 36and 38 is zero, as is the potential across the output terminals 40 and42. With the switches 24, 26 and 28 remaining in the stated position, itmay be seen that as the slider 34 is moved along the slidewire resistor32 towards the upper end thereof, the potential across the voltagedividing resistors 36 and 38 is increased up to the maximum of thepotential developed across the resistor 16 in the range determiningcombination. As the poten tial across the voltage dividing resistors 36and 38 goes up the portion developed across the resistor 38 similarlyincreases. The purpose of the voltage dividing resistors 36 and 38 is topermit the selecting elements of the circuit to be operated at such ahigher voltage level that the significance of contact and thermalpotentials developed therein is oh viated, while yet permitting theoutput voltage across the terminals 40 and 42 to be relatively small,for example in the millivolt range. To accomplish this, it may bereadily apparent that the resistor 36 might well be relatively large ascompared to the resistor 38.

When the slider 34 has reached the upper limit of the slidewire resistor32, in order to further increase the output or reference signal, itbecomes necessary to actuate the switches to introduce the second rangeof signals. Accordingly, the switches 24, 26 and 2 8 are actuated tomove the movable elements thereof to the next fixed contact. Thus, themovable element 24 will he in engagement with the fixed contact 24d ofthe switch 24, the movable contact 26 will be in engagement with thefixed contact 26d of the switch 26 and the movable contact 28 will be inengagement with the fixed contact 28d of the switch 28. With theswitches in this position, the movable contact 24 of the switch 24 isconnected to the lead 20 which is, in turn, connected to the junctionbetween the range determining resistors 12 and 14. The movable contacts26 of the switch 26 is connected to the lead 22 which is, in turn,connected to the junction between the range determining resistors 14 and16. In this position the potential developed across the rangedetenrnining resistor 14 is applied across the slidewire resistor 32 andthe potential developed across the range determining resistor 16 isapplied between the lower terminal of the slidewire resistor 32 and thecommon lead 8. Inasmuch as the movable contact 28 of the switch 28 isconnected, through the fixed contact 28d, to the lower end of theslidewire resistor 32, the potential developed across the rangedetermining resistor 16 is thereby applied across the voltage dividingresistors 36 and 38. This potential is identical to the potentialapplied across the resistors 36 and 38 when the three switches wereconnected in their lowermost position and the slider 34 was at the upperlimit of its travel along the slidewire 32. In this position it will benoted that no connection is made to the slider 34. Thus, the slider 34may be moved along the slidewire 32 without influencing the potentialacross the voltage dividing resistors 36 and-38, hence, withoutinfluencing the output potential across the terminals 40 and 42. Underthese conditions the slider 34 may be returned to the lower end of theslidewire 32 in readiness for an adjustment of the output potentialthrough the middle range of signals.

When the slider 34 has been returned to the lowermost position along theslidewire 32 the switches 24, 26, and 28 may then be moved to the nextsuccessive position, that is, to the fixed contact 0 of each of theswitches. Because of the particular interconnection of the fixedcontacts of the switches 24 and 26, no change in the potentials appliedto the movable contacts 24f and 26 results as a function of the transferfrom the d position to the 0 position. That is, the potential developedacross the range determining resistor 16 is applied between the lowerend of the slidewire resistor 32 and the common lead 8. Similarly, thepotential developed across the range determining resistor 14 is appliedacross the terminals of the slidewire resistor 32. With the slider 34 atthe lower extremity of the slidewire 32 the potential applied across thetwo voltage dividing resistors 36 and 38 hence the two output terminals40 and 42 is unchanged as a result of the transfer of the movablecontacts of the switches 24, 26 and 28 to the 0 position of each of theswitches, respectively. The only change that has taken place as a resultof that transfer is that the movable contact 28 of the switch 28 isconnected, through the 0 contact, to the slider 34 on the slidewire 32.With the switches in this position the slider 34 may now be advancedtoward the upper end of the slidewire 32 producing a progressiveincrease in the potential applied across the two voltage dividingresistors 36 and 38 until the slider 34 has reached the upper limit ofthe slide- Wire 32. At that point the potential across the two voltagedividing resistors 36 and 38 is equal to the sum of the potentialsacross the two range determining resistors 14 and 16.

When the slider 34 has again reached the upper limit of the slidewireresistor 32, in order to still further increase the output or referencesignal, it becomes necessary to again actuate the switches to introducethe next range of signals. As before the switches 24, 26 and 28 areactuated to move the movable elements thereof to the next fixed contact.Thus the movable contact 24 will be in engagement with the fixed contact24b, the movable 26 will be in contact with the fixed contact 26b, andthe movable contact 281 will be in engagement with the fixed contact28b. With the switches in this position, the movable contact 24 of theswitch 24 is connected to the lead .18 which is, in turn, connected tothe junction between the range determining resistors 10 and 12. Themovable contact 26 of the switch 26 is connected to the lead 20 whichis, in turn, connected to the junction between the range determiningresistors 12 and 14. In this position, the potential developed acrossthe range determining resistor 12 is applied across the slidewireresistor 32 and the sum of the potential developed across the rangedetermining resistors 14 and 16 is applied between the lower terminal ofthe slidewire resistor 32 and the common lead 8. Again, since themovable contact 28f of the switch 28 is connected, through fixedcontacts 28b, to the lower end of the slidewire resistor 32, the sum ofthe potential developed across the range determining resistors 14 and 16is thereby applied across the voltage dividing resistors 36 and 38. Thispotential is identical to the potential applied across the resistors 36and 38 when the movable contacts of the three switches were engaged withthe fixed contact 0 of each of the switches, respectively and the slider34 was at the upper limit of its travel along the slidewire 32. Asbefore, in this position there is no connection made to the slider 34.Thus, the slider 34 may be moved along the slidewire 32 withoutinfluencing the potential established across the voltage dividingresistors 36 and 38-, hence without influencing the output potentialacross the terminals 40 and 42. Here, too, the slider 34 may be returnedto the lower end of the slidewire 32 in readiness for an adjustment ofthe output potential through the upper range of signals.

When the slider 34 has been returned to the lowermost position alongwith slidewire 32, the switches 24, 26 and 28 may then be moved to theupper most position, that is, to the fixed contact a of each of theswitches, respectively. Here again, because of the particularinter-connection of the fixed contacts of the switches 24 and 26, nochange in the potentials applied to the movable contacts 24 and 26fresult as a function of the transfer from the b position to the aposition. That is, the potential developed across the range determiningresistors 14 and 16 is applied between the lower end of the slidewireresistor 32 and the common lead 8. Similarly, the potential developedacross determining resistor 12 is applied across the terminals of theslidewire resistor 32. With the slider 34 at the lower extremity of theslidewire 32, the potential applied across the two voltage dividingresistors 36 and 38, hence, the two output terminals are 40 and 42 isthen changed as a result of the transfer of the movable contacts of theswitches 24, 26 and 28 to the a position of each of the switches,respectively. Again, the only change that has taken place as a result ofthat transfer is that the movable contact 28f of the switch 28 isconnected, through the a contact, to the slider 34 of the slidewire 32.With the switches in that position, the slider 34 may again be advancedtoward the upper end of the slidewire 32 producing a progressiveincrease in the potential applied across the two voltage dividingresistors 36 and 38 until the slider 34 has reached the upper limit ofthe slidewire 3-2. At that point the potential across the two voltagedividing resistors 36 and 38 is equal to the sum of the potentialsacross the three range determining resistors 12, 14 and 16. That signalis then the maximum signal obtainable with the illustrated circuit.

In the circuit and operation thus far described it may be seen that thesecond and fourth switch positions, that is the positions indicated bythe fixed contacts 11 and d, respectively are hold positions whereby theoutput of the system is independent of the position of the slider 34along the slidewire resistor 32 whereby the slider 34 may be returned toits standing position between range changes without producing aso-called bump or discontinuity in the progressively adjustabel outputsignal.

It will also be appreciated from an examination of the ilustratedembodiment of the invention that the continuous adjustability of thesystem is equally operable in the reverse direction, that is, startingwith the maximum signal, the signal may be progressively decreasedthrough the several ranges with the intermediate hold positions asbefore.

What is claimed is:

1. An adjustable reference signal circuit comprising a power supplycircuit having a pair of output leads, a plurality of range determiningresistors connected in series 'between said leads, an adjustablepotentiometer having a slidewire resistor and a slider, first switchmeans for selectively connecting said slidewire resistor across aselected one of said range determining resistors and having a pluralityof pairs of switch positions each of which pairs is individual to adiiferent signal range, an output circuit, a further switch meansconnected between said first switch means and said output circuit, saidfurther switch means including two sets of fixed contacts arrangedalternately, one set of said fixed contacts being connected to saidslider, the other set of said fixed contacts being connected to saidfirst switch means whereby to by-pass said potentiometer when saidfurther switch means is closed on said other set of fixed contacts onalternate positions of said further switch means, and means for gangingsaid further switch means to said first switch means to cause saidfurther switch means to close on a contact of said one set when saidfirst switch means has one of the positions of each of said pairs ofpositions, and to close on a contact of said other set when said firstswitch means has the other of the positions of each of said pairs ofpositions, said alternate positions of said further switch means atwhich the latter is closed on said other set of contacts comprising holdpositions between range changes whereat said slider may be returned to astarting position on said slidewire resistor without affecting an outputsignal on said output circuit.

2. An adjustable reference signal circuit comprising a power supplycircuit having a pair of output leads, a plurality of range determiningresistors connected in series between said leads, an adjustablepotentiometer having a slidewire resistor and a slider, first switchmeans having a plurality of switch positions for selectively connectingsaid slidewire resistor across a selected one of said range determiningresistors, an output circuit, and a second switch means having aplurality of switch positions corresponding in number to the number ofswitch positions of said first switch means, said second switch meanshaving a first set of switch positions whereat said slider is connectedto said output circuit, said second switch means having a second set ofswitch positions arranged intermediate said first switch positions ofsaid second switch means whereat said output circuit is connected tosaid first switch means by-passing said potentiometer, said plurality ofswitch positions of said first switch means including a first switchposit-ion representative of a first signal range, the remainder of saidplurality of switch positions of said first switch means being groupedin pairs of switch positions, and each of said pairs of switch positionsbeing representative of a different signal range, said first switchmeans and said second switch means being ganged together for conjointoperation, said intermediate switch positions of said second switchmeans and the corresponding switch positions of said first switch meansconstituting hold positions whereat said slider may be returned to astarting position on said slidewire resistor without affecting thesignal on said output circuit determined by the switch position of saidfirst switch means relative to said range resistors.

3. An adjustable reference signal circuit comprising a power supplycircuit having a pair of output leads, a plurality of range determiningresistors connected in series between said leads, an adjustablepotentiometer having a slidewire resistor and a slider, first switchmeans having a plurality of switch positions for selectively connectingsaid slidewire resistor across a selected one of said range determiningresistors, an output circuit having a pair of leads one of which isconnected to a corresponding one of said power supply output leads, andsecond switch means having a plurality of switch positions correspondingin number to the number of switch position of said first switch means,said second switch means having a first set of switch positions whereatsaid slider is connected to said output circuit, said second switchmeans having a second set of switch positions arranged intermediate saidfirst switch positions of said second switch means whereat said outputcircuit is connected to said first switch means bypassing saidpotentiometer, said plurality of switch positions of said first switchmeans including a first switch position representative of a first signalrange, the remainder of said plurality of switch positions of said firstswitch means being grouped in pairs of switch positions, and successiveone of said pairs of switch positions being representative of successivesignal ranges, said first switch means and said second switch meansbeing ganged together for conjoint operation, said intermediate switchpositions of said second switch means and the corresponding switchpositions of said first switch means constituting hold positions whereatsaid slider may be returned to a starting position on said slidewireresistor without affecting the signal on said output circuit determinedby the switch position of said first switch means relative to said rangeresistors.

4. The invention as set forth in claim 3 wherein said first switch meansincludes a first switch member and a second switch member, each of saidswitch members including a plurality of fixed contacts corresponding innumber to said plurality of switch postions, each of said switch membershaving a movable contact, said movable contact of said first switchmember being connected to one end of said slidewire resistor, and saidmovable contact of said second switch member being connected to theopposite end of said slidewire resistor.

5. The invent-ion as set forth in claim 4 wherein said second set ofswitch positions of said second switch means is connected to the movablecontact of said second switch member of said first switch means.

6. The invention as set forth in claim 3 wherein said output circuitincludes voltage dividing resistors to substantially reduce themagnitude of the output signal.

7. An adjustable reference signal circuit comprising a power supplycircuit having a first and a second output lead, a plurality of rangedetermining resistors connected in series between said leads, anadjustable potentiometer having a slidewire resistor and a slider, firstswitch means having a plurality of switch positions for selectivelyconnecting said slidewire resistor across a selected one of said rangedetermining resistors said first switch means including a first switchmember and a second switch member, said first and second switch memberseach having a movable contact and a plurality of fixed contactscorresponding in number to said plurality of switch positions of saidfirst switch means, means connecting said movable contact of said firstswitch member to one end of said slidewire resistor, means connectingsaid movable contact of said second switch member to the opposite end ofsaid slidewire resistor, means connecting a fixed contact of said secondswitch member to said second output lead, the remainder of said fixedcontacts of said second switch member being connected in pairs, meansconnecting successive pairs of said fixed contacts of said second switchmember to successive junctions between said range determining resistors,respectively, means connecting a first fixed contact of said firstswitch member to the junction between a first and a second one of saidrange determiningresistors, the remainder of said fixed contacts of saidfirst switch member being connected in pairs, means connectingsuccessive pairs of said fixed contacts of said first switch member tosuccessive junctions between said range determining resistors,respectively, subsequent to said junction to which said first fixedcontact of said first switch member is connected, an output circuit, asecond switch means including a further switch member having a movablecontact and a plurality of fixed contacts corresponding in number to thenumber of switch positions of said first switch means, means connectingalternate ones of said fixed contacts of said further switch membertogether and to said slider, means connecting the fixed contacts of saidfurther switch member intermediate said alternate contacts together andto the movable contact of said second switch member, and meansconnecting said movable contact of said further switch member to saidoutput circuit.

8. An adjustable reference signal circuit comprising a power supplycircuit having a first and a second output lead, a plurality of rangedetermining resistors connected in series between said leads, anadjustable potentiometer having a slidewire resistor and a slider, firstswitch means having a plurality of switch positions for selectivelyconnecting said slidewire resistor across a selected one of said rangedetermining resistors, said first switch means including a first switchmember and a second switch member, said first and said second switchmembers each having a movable contact and a plurality of fixed contactscorresponding in number to said plurality of switch positions of saidfirst switch means, means connecting said movable contact of said firstswitch member to one end of said slidewire resistor, means connectingsaid mova'ble cntact of said second switch member to the opposite end ofsaid slidewire resistor, means connecting a fixed contact of said secondswitch member to said second output lead, the remainder of said fixedcontacts of said second switch member being connected in pairs, meansconnecting successive pairs of said fixed contacts of said second switchmember to successive junctions between said range determining resistors,respectively, means connecting a first fixed contact of said firstswitch member to the junction between a first and a second one of saidrange determining resistors, the remainder of said fixed contacts ofsaid first switch member being connected in pairs, means connectingsuccessive pairs of said fixed contacts of said first switch member tosuccessive junctions between said range determining resistors,respectively, subsequent to said junction to which said first fixedcontact of said first switch member is connected, an output circuit, asecond switch means including a further switch member having a movablecontact and a plurality of fixed contacts corresponding in number to thenumber of switch positions of said first switch means, means connectingalternate ones of said fixed contacts of said further switch membertogether and to said slider, means connecting the fixed contacts of saidfurther switch member intermediate said alternate contacts together andto the movable contact of said second switch member, and meansconnecting said movable ontact of said further switch member to saidoutput circuit, said output circuit including voltage dividing resistorsto substantially reduce the magnitude of the output signal.

References Cited by the Examiner UNITED STATES PATENTS 2,572,545 10/1951Walker 323-79 2,707,222 4/1955 Brown et al 323-94 2,774,934 12/ 1956Gitzendanner 32396 2,935,368 5/1960 Moseley 32379 3,015,790 1/1962Eisamen et al 323 JOHN F. COUCH, Primary Examiner.

LLOYD MCCOLLUM, Examiner.

D. L. RAE, A. D. PELLINEN, Assistant Examiners.

1. AN ADJUSTABLE REFERENCE SIGNAL CIRCUIT COMPRISING A POWER SUPPLYCIRCUIT HAVING A PAIR OF OUTPUT LEADS, A PLURALITY OF RANGE DETERMININGRESISTORS CONNECTED IN SERIES BETWEEN SAID LEADS, AN ADJUSTABLEPOTENTIOMETER HAVING SLIDEWIRE RESISTOR AND A SLIDER, FIRST SWITCH MEANSFOR SELECTIVELY CONNECTING SAID SLIDEWIRE RESISTOR ACROSS A SELECTED ONEOF SAID RANGE DETERMINING RESISTORS AND HAVING A PLURALITY OF PAIRS OFSWITCH MEANS POSITIONS EACH OF WHICH PAIRS IS INDIVIDUAL TO A DIFFERENTSIGNALS RANGE, AN OUTPUT CIRCUIT, A FURTHER SWITCH MEANS CONNECTEDBETWEEN SAID FIRST SWITCH MEANS AND SAID OUTPUT CIRCUIT, SAID FURTHERSWITCH MEANS INCLUDING TWO SETS OF FIXED CONTACTS ARRANGED ALTERNATELY,ONE SET OF SAID OF FIXED CONTACTS BEING CONNECTED TO SAID SLIDER, THEOTHER SET OF SAID FIXED CONTACTS BEING CONNECTED TO SAID FIRST SWITCHMEANS WHEREBY TO BY-PASS SAID POTENTIOMETER WHEN SAID FURTHER SWITCHMEANS IS CLOSED ON